The low frequency motions of solvated Mn(ii) and Ni(ii) ions and their halide complexes

Abstract

We have investigated the low frequency (30-350 cm(-1)) spectra of solvated MnCl2, MnBr2, NiCl2, and NiBr2. Using a chemical equilibrium model in combination with principal component analysis, we were able to dissect the spectra into molar extinction coefficients due to the solvated ions and - for MnCl2, MnBr2, and NiCl2- to extract information on the ion pair spectra. The deduced anion spectra (calculated as MnCl2-MnBr2 and NiCl2-NiBr2) are very similar and nearly identical to the anion spectra observed for LaCl3-LaBr3. The differences between the cationic contributions MnCl2-NiCl2 and MnBr2-NiBr2 indicate that the solvated cation spectra can be understood in terms of distinct resonances of the octahedrally solvated cation complex that are red-shifted for Mn(2+) compared to Ni(2+). The description of the full extinction spectra requires the introduction of an additional resonance at a center frequency of around 130 cm(-1) that we tentatively assign to hydration water. Cooperative effects are small and are reflected in a change in the band intensity. However, the center frequencies of the observed modes remain unchanged when exchanging the counter ion. Analysis of the ion pair extinction spectra supports contact ion pair formation for MnBr2 and NiCl2 and MnCl2.

Solvation Dynamics of Trimethylamine N-Oxide in Aqueous Solution Probed by Terahertz Spectroscopy

“…This idea is supported by molecular dynamics simulations 45,46 and linear infrared spectroscopy 24 that shows that the hydrophilic entity of TMAO forms two to three strong hydrogen bonds to water. In comparison, the dynamic solvation size for Ni 2+ and Mn 2+ is found to be 5 -6 36 and ≈ 5 for HCl and HBr. 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 spectroscopy 16,17 is not observed in the THz/FIR spectral range.…”

“…In contrast to salts we find no evidence for high frequency water absorption due to the water hindered librational mode around 600 cm -1 . 36 This band seems to be suppressed or shifted out of our experimentally accessible range.…”

“…For further analysis we follow the approach outlined in the previous paper by Sharma et al 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 PC1 PC2 100 200 300 400 1500 1000 500 0 500 1000 Wavenumber in cm 1 Loading vector in cm 1 10 Figure 5: Primary (blue) and secondary principal component (purple) obtained in a singular value decomposition for the THz/FIR spectra of TMAO. The inset shows the concentration dependency of the corresponding scores together with a fit assuming the formation of solute-solvent aggregates with up to four water molecules.…”

“…We have used damped harmonic oscillator line shapes. 36 In addition, a contribution n hydration was added accounting for the hydration water. Values in brackets give the 95% confidence interval in units of the last digit.…”

“…We added the absorption of bulk water 36 to ∆α in order to obtain the absorption of the sample solution:…”